In vehicles with diesel engines, an oxidation catalyst is typically used to reduce the level of carbon monoxide, nitrous oxide and unburnt hydrocarbons present in the engine exhaust gas. Early diesel oxidation catalysts were composed of platinum on a high surface area support, such as alumina, and were generally operated at temperatures up to 500 to 600° C. More recently, diesel oxidation catalysts have been required to operate at higher temperatures, typically of the order of 700 to 800° C., in order to regenerate the particulate filter that is conventionally located downstream of the oxidation catalyst. It is known that mixed platinum and palladium catalysts offer improved thermal stability as compared with platinum alone and hence the catalyst industry has shifted almost completely to PtPd-based diesel oxidation catalysts. However, currently available Pt/Pd-based oxidation catalysts suffer from the problems of poor alloying between the platinum and palladium and a tendency for the size of the metal particles to grow both as the platinum concentration is increased and during use. Both of these factors limit the performance of the catalyst and the possibility for further enhancements in thermal stability.
There is therefore significant interest in developing diesel oxidation catalysts with improved properties and particularly with improved initial activity, improved thermal stability, controlled and stable metal particle size and reduced aging.
According to the present invention, a novel supported Pt/Pd alloy catalyst has now been produced in which the metal alloy particles exhibit a unique combination of small size, controlled and uniform shape, homogeneous dispersion and high surface area.